1. Field of the Invention
The present invention relates to an image processing unit and image processing method of a digital still video camera that carries out exposure control, gain control and white balance control using coded data formed by coding image signals output from an image pickup apparatus.
2. Description of Related Art
Japanese patent application laid-open No. 4-298170/1992 discloses a technique about a digital still video camera with a function of coding, through orthogonal transformation, image signals acquired by an image pickup device such as a CCD (charge-coupled device). This technique utilizes DC (direct current) component data obtained by the orthogonal transformation in order to control exposure.
FIG. 16 is a block diagram showing a configuration of a digital still video camera disclosed in the above-mentioned Japanese patent application laid-open No. 4-298170. In FIG. 16, the reference numeral 101 designates a diaphragm for spatially controlling the amount of light entering the camera; 102 designates a shutter for temporally controlling the amount of the incident light; 103 designates a CCD for converting the received incident light to electrical signals; 104 designates a Y/C separator for generating from the image signals output from the CCD 103 a luminance component and color components to be output separately; 105 designates a CCD driver for supplying the CCD 103 with drive signals for driving it; 106 designates an A/D converter for converting the analog luminance signal and the color signals output from the Y/C separator 104 into digital image data; 107 designates a high-speed buffer memory for temporarily storing the image data output from the A/D converter 106; 108 designates a data compressor/decompressor that reads from the high-speed buffer memory 107 the image data, divides the image data of an amount of a single screen into a plurality of blocks, and carries out data compression by coding through quantization of frequency information obtained by orthogonal transformation of each block, or data expansion by the reverse transformation; 109 designates a recording memory for recording the data compressed image data through the data compressor/decompressor 108; 110 designates a D/A converter for converting the image data read from the high-speed buffer memory 107 into an analog signal; and 111 designates a system controller that supplies the foregoing blocks with timing signals for controlling their operation, and outputs controlling signals for controlling the diaphragm 101 and the shutter 102 by reading as photometric data the DC component data of each block after the orthogonal transformation in the data compression process in the data compressor/decompressor 108.
Next, the operation of the conventional digital still video camera will be described.
In this conventional digital still video camera, the system controller 111 reads block by block the DC component data of each block after the orthogonal transformation in the data compression process by the data compressor/decompressor 108, and carries out the exposure control by controlling the driving of the diaphragm 101 and the shutter 102 by using the read data as the photometric data, thereby achieving the right exposure to a subject of the camera.
The conventional image processing unit with the foregoing structure employs the orthogonal transformation as its coding scheme. Thus, calculating and coding the photometric data on the real time basis requires a real time processing of a series of coding sequences. This, however, has a problem of complicating the control and increasing the circuit scale because the orthogonal transformation coding is one of the variable length coding methods.
Furthermore, to use the photometric data to implement gain correction of the digital data of the picked-up image, it is necessary to hold besides the coded data the DC components, or to regenerate the DC components by decoding the coded data, which presents a problem of increasing the processing procedure.
In addition, since the gain correction of the digital data obtained by coding the picked-up image cannot be provided directly to the coded data, the coded data must be decoded once, and then subjected to the sequential processing. This presents another problem of hindering the high speed processing.
Moreover, since the coding based on the orthogonal transformation is one of the variable length coding methods, the control such as calculating addresses for reading the coded data during the decoding becomes complicated, and this presents a problem of reducing the processing speed and increasing the circuit scale.
Furthermore, coding into a single color signal component the image output from the image pickup device, which includes a plurality of color filters, poses a problem of degrading the accuracy of the photometric data. This is because the color modulation components are each processed as a resolution component, which increases the quantization error due to the coding.
In addition, it is necessary for the white balance processing, which corrects sensitivity errors in individual colors of the image output from the image pickup device including a plurality of color filters, to have coding circuits of the number of the color components; and this presents a problem of increasing the circuit scale.
Moreover, to select a block used for determining the photometric data in desired conditions, it is necessary for the processing of deciding the conditions to utilize data in the process of coding rather than the coded data themselves. This presents another problem of increasing the processing procedure.
Finally, to select a block used for determining the photometric data in desired conditions, the block to be used for generating the photometric data cannot be selected by a simple processing because the processing, which carries out the gain correction of the digital data obtained by coding the picked-up image, employs the variable length coding. This presents still another problem of complicating the processing.